Automatic elemental analyzers are based on the principle of dynamic flash combustion of a sample followed by the elimination of the excess oxygen, the separation of the gases produced by the combustion, the passage of the gases produced through the packed column of a gas chromatographic column and subsequent quantitative determination with a thermal conductivity detector.
The accuracy and reproducibility of the elemental analysis is dependent on a number of factors, one of which is the completeness of the Combustion process. A minimum quantity of Oxygen is necessary to ensure complete combustion. The minimum quantity of Oxygen required depends on the nature of the sample matrix, the sample weight and the adjuvant to the combustion itself. The use of a large excess of Oxygen favors the completeness of the combustion process but results in undesirable consequences such as reduction in the lifetime of the expensive catalysts used in the elimination of the excess Oxygen downstream of the combustion, and an increase in the downtime of the instrument.
Furthermore, the optimization of the combustion process also relies on the capability to control the duration of the Oxygen admission. For identical Oxygen volume requirements, some sample matrices require a very sharp and short injection time, whilst others require that the Oxygen be administered at a slower rate.
The commercially available devices, to date, are based on the admission of fixed volumes of Oxygen from reservoirs of defined volumes, swept by a carrier gas. Whenever different volumes of Oxygen are required, the reservoirs must be manually exchanged, thus making the optimization of the oxygen quantity, labor intensive. Additionally none of these devices have the capability to control the rate of the Oxygen admission. These devices rely on the admission of a significant excess of Oxygen in order to endeavor to achieve complete combustion, however, even with a large excess of Oxygen, complete combustion is not necessarily achieved.
A recent application of the same applicant (Italian Patent Application BS97A000033) describes an arrangement which provides a facility to admit varying quantities of Oxygen. An Oxygen flow maintained constant by a mass flow controller is allowed to pass into the combustion chamber of the system, for periods of time which can be defined by the operator. In this arrangement, the admission of Oxygen occurs whilst the flow of carrier gas is interrupted. The flow of carrier gas is restored after the admission of Oxygen, hence these two gases flow alternately in the carrier line.